Spraying a material onto the surface of a rotating body is commonly done. For example, interior surfaces of metal beverage cans are coated to preserve the flavor of the contents from being changed due to contact with a metal surface. A variety of spray systems have been developed over the years. In the can industry, can interiors are sprayed using one or more spray applicator devices having one or more nozzles positioned near the can interior. Material is sprayed onto the can surfaces while the can is rotated. Can surfaces may include interior and exterior surfaces.
In many applications it is important to assure that the entire surface is coated. The amount of material that is applied to a surface is usually measured in terms of coating weight. In an ongoing effort to reduce costs, coating weights have also been reduced. However, lower coating weights necessitate tighter control over the coating process. There are many process variables that affect coating weight, including temperature, pressure, viscosity, spray duration, nozzle flow rate and pattern control, and spray applicator position. In typical known rotating coating application systems, each deposition of material onto the circumferential surface of the container body is called a wrap. In a known can coating system, a can may be coated with a single wrap or two or more wraps.
The amount of material that is applied to a rotating surface is a function of the above noted process variables, the number of wraps, and also the rotation speed of the surface. If the rotation speed were always a known constant, then the amount of material applied to the surface could be better controlled within the ability of the manufacturer to control the other process variables. But in practice it is very difficult to maintain a constant speed of rotation of the surface being sprayed. As a result, the other process variables noted above have a much greater impact on the coating weight and completeness of each wrap. For example, the actual spray duration can have a major impact on the amount of coating material applied to the rotating surface as a function of the speed of rotation. Spray duration refers to the time duration that coating material impinges the surface being sprayed. Spray duration is thus affected by flow characteristics of material through the spray application device, material transport times and spray device turn on and turn off time delays. The turn on time delay refers to the time delay between the command to turn the spray application device on via a first trigger signal to the spray application device and the actual time that material begins to impinge the surface. Turn off delay refers to the time delay between the command to turn the spray application device off via a second trigger signal to the spray application device and the actual time that material stops impinging on the surface. If the rotation speed is not constant, the spray duration time greatly impacts the completeness of the wraps and the distribution of coating weight applied during each wrap.
In known can spraying systems, can rotation is effected by a suitable drive mechanism that spins the can or surface at an expected rate. There is a wide variety of such drive mechanisms, including but not limited to belt drive systems and vacuum chuck systems. Even though the drive motor or mechanism can be fairly well controlled for rotation speed, such speed data does not necessarily translate into a known rotation speed of the surface being sprayed. For example, in a belt drive system, a can is rotated by contact with a rotating belt. However there can be significant slippage between the belt and can. In vacuum chuck systems there may also be slippage between the can and the chuck. Moreover, precise control of the drive mechanism speed of rotation comes at a cost that adds to the overall cost of the spray application system.
Prior to our invention it is believed that rotating spray application systems have not taken into account the actual speed of rotation of the surface being sprayed. Rather, prior efforts have been directed to controlling the other process variables that affect coating weight, or attempting indirectly to control can rotation speed by controlling rotation speed of the drive mechanism. However, because actual surface rotation speed varies, and further because there are so many additional process variables that affect coating weight as a function of rotation speed, the surfaces must be overcoated to ensure that the requisite number of wraps is achieved. This overcoat of excess coating material can be on the order of about fifteen to about thirty percent or more, and results in a substantial waste of material being sprayed. Overcoat conditions also slow down the overall can processing time since more time is required to spray each can. Additionally, due to the overall lack of tight control of the various process parameters, known spray applications systems necessitate costly inspection requirements to visually or otherwise verify the quality of the coating wraps applied to the surface.
The need exists therefore to provide process and apparatus for applying material to a surface of a rotating body that overcomes or diminishes the aforementioned limitations of known systems.